Commercially popular, successful incremental printing systems primarily encompass inkjet and dry electrographic—i.e. xerographic—machines. (As noted above, the latter units are only partially incremental.) Inkjet systems in turn focus mainly upon on-demand thermal technology, as well as piezo-driven and variant hot-wax systems.
On-demand thermal inkjet, and other inkjet, techniques have enjoyed a major price advantage over the dry systems—and also a very significant advantage in electrical power consumption (largely due to the energy required to fuse the dry so-called “toner” powder into the printing medium). These advantages obtain primarily in the market for low-volume printing, and for printing of relatively short documents, and for documents that include color images or graphics.
A “dedicated computer” such as mentioned above may take any of a great variety of forms, including one or more application-specific integrated circuits (“ASICs”). Another option, merely by way of example, is one or more partially or completely preprogrammed patch boards such as raster image processors (“RIPs”).
Pagewide arrays have been commercialized for years. In the past, however, such arrays have been somewhat disfavored because—in comparison with scanning printers—as a practical matter they offer relatively little opportunity to mitigate end-effects of individual dice through multipass printing.
To look at this from a somewhat opposite perspective, multipass printing is itself undesirable because it is time consuming; and one especially important appeal of pagewide arrays is printing speed or so-called “through-put”. Speed of printing, together with cost, is a major driver of competition in the incremental-printing field.
Hence, minimizing the number of printing passes in a pagewide system is extremely important; however, adverse image-quality effects that arise at and near the end of each individual inkjet die in a pagewide array are also extremely important. These adverse effects tend to under-cut the principal advantages and the strong commercial appeal of pagewide printing.
As always, a critical challenge in pagewide printing machines is this tension between design to minimize the number of passes and design to maintain excellent image quality. The present invention answers this challenge by following a different path to high image quality.
More specifically, one obstacle to best quality in a pagewide machine is that a large number of variables affects quality at each point in an image:
First, inkjet dice are not uniform—neither along the length of each die, nor as among the plural dice that make up a single pagewide array. Therefore different imaging properties arise conspicuously in high-volume use of any pagewide array. Due to these nonuniformities, as will be detailed and explained in a later section of this document, typical pagewide arrays are found to print so-called “light-color bands” (in this document used interchangeably with “light-area bands”) along the direction of motion of the printing medium, beneath the arrays.
Second, color printing is expected to perform properly over a very great range of tonal values in the images to be printed for end-customers or other end-users. That is to say, the tonal operating range is not subject to selection by the designer or the printer—or by the printer operator, either. Therefore the light-color banding cannot be avoided by choosing tonal operating range.
Third, from the viewpoint of a system designer, the images themselves likewise must be considered arbitrary, also not subject to selection. In other words, both the designer and the machine operator must take every image that appears in the print queue as they find it. Most particularly, the positional distribution of tonal values within every image is not under control of the designer, the operator or the machine itself in the field. Therefore the light bands also cannot be removed by shifting the image relative to the printing system.
Fourth, as a consequence the positional distribution of tones is likewise not controllable in relation to the individual dice—or, most particularly, in relation to either (1) position alone each die, or (2) specific micro-location of internal portions of the die ends. Once again the machine is expected to somehow do the best possible job of rendering every tone value that arrives for printing, regardless of interactions with the other factors stated above.
This best-possible rendering is required, or at least very importantly desired, even though detailed image features may (and probably will) require different treatment depending on the part of the image which contains that tone value and those image features. The implication of this requirement, therefore, is that the original machine design should somehow accommodate the unknown, unknowable relationships among the tone, the feature, and most specifically their positions between or within the die ends.
Fifth, preferably all this optimization should avoid the high costs and computation times inherent in previous solutions that required, e.g., high-resolution scanners built into the printing machine or separately deployed. Such equipment also must be interfaced with the computing apparatus that controls the printer, and in general this precludes or at least discourages use of third-party scanners whose operating parameters are potentially and in fact usually alien to the computer system. This is an unfortunate requirement, since such third-party scanners are often available on the open market and often (being necessarily competitive) very economical.
Sixth, and perhaps even more troublesome than other factors discussed above, we have found that even when a high-resolution scanner is used to guide the band-hiding operation of the printer, optimization is less than ideal. That is, resultant band-hiding as then perceived by human users is not very good—or not as good as desired. Perceptual mismatch diverges significantly from straightforward machine-based tonal analysis. The divergence can be attributed to nonlinearities in both the perceptual and machine domains; however, perhaps the former are larger.
Seventh, although various former procedures are known for controlling incremental printers in response to human input, those former methods fail to provide a satisfactory optimization for light-color banding in pagewide arrays. Specifically, past procedures used in operator/machine dialogs relate to simpler adjustments that involved fewer variables.
For instance these earlier methods are for aligning printheads to one another, or for matching inking levels. Therefore those methods first print a set of test patterns side by side, representing e.g. various candidate print-head-alignment relationships, or plural candidate color-matching relationships. An operator selects a candidate that forces two lines of different colors into alignment; or one that makes two colors appear to match in some simple regard, usually one-dimensional—e.g. intensity or saturation.
As suggested above by the first four discussions of printing variables, the problem addressed by this present invention is more complicated. There is no single variable domain in which a match-up can be made to resolve the multidimensional determination in this environment.
Yet another consideration is that inkjet printing, in general, benefits from linearization (at least moderately accurate linearization) of the relationship between tonal values specified in the input image data and human-perceived tonal values in the printed output image. Extremely precise linearization is not a requirement; yet some photographers—even some amateurs—are sensitive to nonuniform reproduction of tonal increments, and to other contrast anomalies. Some prior efforts to correct die-generated artifacts may simply overlay corrective colorant patterns onto already-linearized image regions, thus potentially generating a new and different kind of colorant error.
Conclusion—In summary, achievement of uniformly excellent inkjet printing, particularly using pagewide arrays, continues to be impeded by the above-mentioned problems of light-area, light-color bands appearing at or near seams between adjacent printing dice—due to printing nonuniformities at the seams. As shown above, these variations are aggravated by a very great range of tonal values to be printed, and the fact that such tones are free to occur at essentially any position in an image—and any position relative to the seams.
Other adverse factors include the cost of adequate scanning equipment, poor perceptual results even when good scanners are used, and too many variables for the simple match-ups used in prior perception-based methods—as well as failure to integrate corrections into the overall linearization scheme of the inkjet printing process. Another adverse effect may be imprecision of printing-medium advance in the transverse direction, between printing passes. Thus very important aspects of the technology used in the field of the invention remain amenable to useful refinement.